The present invention relates to a photomask used in an exposing process for manufacturing a semiconductor device, and more specifically, to a photomask that includes an assistant pattern formed in an open region around the main pattern.
In photolithography for manufacturing a semiconductor device, a photomask is used to form a pattern on a semiconductor substrate. The photomask has a mask pattern for forming various components of the semiconductor device. As the semiconductor device becomes more integrated, the feature size of the mask pattern becomes smaller.
When the feature size of the mask pattern reaches a resolution limit of an exposer, it becomes difficult to transfer a desired pattern on a substrate due to the optical proximity effect.
That is, when the mask pattern is transferred on a substrate by illumination of exposure sources (e.g., KrF excimer laser or ArF excimer laser), the pattern formed on the photomask is not transferred to the substrate uniformly. The pattern is distorted depending on the its location of the pattern. When the light transmitted by a light source passes through the photomask, various optical phenomena occur depending on the location and the shape of the mask pattern so that the light intensity varies depending on the location on the photomask.
In order to solve the above problem, an optical proximity correction (OPC) technology has been used.
The OPC is to calculate statistically or experimentally a relation between an intended pattern and an actual pattern formed in photoresist. The size and shape of the mask pattern can then be adjusted depending on the calculation.
However, when a conventional OPC is used, it is difficult to improve the margin of the depth of focus and the uniformity of a critical dimension (CD) in all of the patterns of the chip. A pattern formed at an outer edge of a cell array is particularly vulnerable to defocus.
FIG. 1 is a diagram illustrating a SEM photograph of contact holes formed in a cell array region when optical proximity correction is performed without an assistant pattern.
Referring to FIG. 1, a contact hole pattern disposed in the outer edge is distorted due to the defocus phenomenon. FIG. 1 shows how the defocus phenomenon changes in the outer edge pattern as the depth of focus changes from −0.2 μm to −0.04 μm.
FIG. 2 is a diagram illustrating an aerial image of the contact hole pattern of FIG. 1 with the best focus.
As shown in an aerial image 12 disposed between contact holes 11 of FIG. 2, when defocus occurs, likelihood of generating bridges between the adjacent contact holes 11 increases significantly.
In order to prevent the generation of bridges, a method of enlarging a hole diameter of the outer edge pattern has been used. However, when the diameter of the contact hole is enlarged, a contact region encroaches into a gate region and may cause a short between the two regions. As a result, the method of enlarging a hole diameter of the outer edge pattern has a limit.
Alternatively to the solution above, a method of inserting an assistant pattern into a photomask may be used. However, it is difficult to obtain a desired outer pattern and a desired process margin by this method.
As a result, a new assistant pattern is required to obtain a desired process margin for forming the outer pattern, which is most vulnerable to the defocus phenomenon.